1. Field of the Invention
The invention relates to production of complex components from metallic or ceramic materials wherein powders are used as the starting materials. The invention also addresses questions concerning shrinkage due to sintering and hot-isostatic pressing.
The invention relates to the further development, perfection and simplification of powder-metallurgical production methods for the production of workpieces of comparatively complex shapes, where the problems of shrinkage during sintering play an important role. The preferred field of application is the component sector in turbine construction.
In the narrower sense, the invention relates to a process for the production of a component. The process includes (a) producing a molding using a pourable metal or ceramic powder as the starting material, by applying the powder, transported by means of a stream of gas, under centrifugal force to the inner wall of a mold which is under reduced pressure and (b) sintering the precompacted body.
2. Discussion of Background
Powders are used as the starting materials in numerous production methods in the metallurgical and ceramics industries. Powder-metallurgical processes have the advantage that virtually any desired shape can be achieved. The intention is to produce finished workpieces by a powder metallurgy process which eliminates some or all of the expensive machining costs. The starting materials in all of the known processes for obtaining net shapes or near-net shapes of the workpieces are slurries (slip, paste) of powders in solvents using a binder. The following additives are used in powder mixtures:
A. water+binder+additive (slip casting, freeze drying); PA1 B. water+cellulose (metal-powder injection molding (MIM) by the Rivers process); and PA1 C. thermoplastics (metal-powder injection molding). PA1 3. The occurrence of binder residues (for example carbon), which, even after "burning out" the binder, remain behind in the workpiece and can impair its composition in an uncontrolled manner. PA1 4. The necessity for fresh selection/fresh development of the binder when changing to other shapes and/or compositions of the workpieces.
With all of these wet-mechanical methods numerous difficulties arise with regard to quality, freedom of shaping, reproducibility and choice of the composition. Such difficulties include the following:
1. Bubble formation when mixing powder with binder and solvent.
2. Restriction of the wall thickness of the workpieces (for example max. 5-10 mm for "MIM"), since otherwise the binder can no longer be completely removed.
In the case of metal injection molding (MIM) a mixture of the metal powder to be compacted is injected into a mold together with a suitable thermoplastic in accordance with the injection molding technology. A summary of the methods for "Metal Injection Molding" is given in a chapter of the Metals Handbook.
A particular problem with this technology is, on the one hand, the fact that in general considerably finer powders have to be employed than is usually the case in powder metallurgy. On the other hand, the organic binder must be removed by a laborious process before the actual sintering process, which leads to a considerable increase in the cost of the process.
The vacuum-molding process, which serves for the production of casting molds from refractory granular mold material, as a rule quartz sand, is known from casting technology. By evacuation of the air from a heap of binder-free sand surrounded by sheeting, a reduced pressure is generated in said sand, as a result of which a compressive pressure is exerted by the adjacent outside gas atmosphere via the sheeting on the loose sand fill. The compressive strains thus caused between the grains prevent the mutual mobility of the latter. As a result, a mechanically strong body of defined shape is formed from a loose heap.
In the production of moldings which are subjected to a subsequent sintering process, the uniformity of the loose powder fill at all points of the molding is extremely important since the local extent of shrinkage, and thus the dimensional accuracy, are a function of the local settled apparent density.
There are processes from the field of powder metallurgy where mixtures of a metal powder with a liquefied organic phase are injected into molds by the injection molding process. After the filling operation, a compact composite of uniform density is formed, from which the organic binder must be removed before the actual sintering process starts.
There are other processes in which essentially dry powders are filled into a mold under vacuum. This operation can, for example, also be supported by a suitable vibration or shaking operation. However, because of the frictional resistance of the powder, there are limits to the complexity of shaping. In addition, there is a risk that the various grain fractions of a powder will demix under the influence of the movement of the powder, especially under the action of vibrations, as a result of which an inhomogeneous sintered compact forms.
With the aid of one process, for example, a molding is produced by a procedure in which a pourable molding composition is fluidized using a transport gas. The molding composition passes into the interior of a mold which is under reduced pressure and which contains suction orifices at certain points for drawing off the transport gas. A substantial part of the description of this process is dedicated to the optimum sizing and arrangement of these suction orifices and to the optimum timing of the injection and suction processes, since both the geometrical arrangement and the timing are of extremely great importance for the production of a molding having a uniform settled apparent density. When the fluidized powder penetrates into the interior of the mold, expansion of the gas occurs along with kinetic acceleration of the powder particles. As a result, powder particles are driven by centrifugal force against the wall of the mold. However, since the wall of the mold is impermeable to gas in substantial sections, only a coating of the wall is achieved by the kinetic energy of the grain particles. Thus, special precautions must be taken in order to prevent premature blocking of the off-gas channels by powder preferentially flying in this direction.
The following publications are cited as representative of the prior art:
GB Pat. Appl. 2088414 PA0 EP Pat. Appl. 0191409 PA0 DE-A-3,101,236 PA0 DE-A-3,128,347 PA0 DE-A-3,128,348 PA0 DE-A-3,542,332
R. Billet, "PLASTIC METALS: From Fiction to Reality with Injection Molded P/M Materials", Parmatech Corporation, San Rafael, Calif., P/M-82 in Europe Int. PM-Conf. Florence I 1982.
Goran Sjoberg, "Powder Casting and Metal Injection Molding", manuscript submitted to Metal Powder Report September 1987.
Henry H. Hausner, "Slip Casting of Metal Powders", in "Perspectives in Powder Metallurgy", Hausner et al., Plenum Press 1967.
The known processes leave something to be desired. There is therefore a need for improvement and further development of the powder-metallurgical/powder-ceramic production methods.